Two types of plants (bright green snake plant / bright green barrel cactus) produce green thorns.
They cannot be obtained as loot, so you have to get involved in the plant-crossing business if you want to manage everything by yourself and begin from the scratch.
Unfortunately, there is no 'deterministic' way of obtaining a barrel cactus seed from crossing due to a glitch. So it is probably easier to work with snake plant.
Of course, buying those seeds from other botanists is the best way to avoid all this plant-crossing business.